A reduction in plantarflexor power during the push-off phase of walking leads to the slowing of preferred walking speed with age, which in turn negatively affects old adults' health and independence. Although commonly implicated, sarcopenia and muscle weakness alone cannot fully explain the reduction in plantarflexor power or accompanying changes in coordination. We postulate that this disconnect arises from age-related changes in Achilles tendon behavior that alter muscle-tendon dynamics during movement. This study tightly integrates novel in vivo imaging, computational modeling, and motion analysis to investigate tendon deformations associated with physiological loading and movement to an unprecedented level of detail. Our overarching hypothesis is that age-related changes in tendon elasticity and inter-fascicle adhesions have a substantial effect on the ability for muscles to generate sufficient plantarflexor power during movement. This study has three aims. The first aim is to determine how advancing age affects the in vivo behavior of the plantarflexor muscles and Achilles tendon during prescribed ankle flexion movements under physiological loading. We will combine high-resolution static MRI, dynamic MRI, and shear wave elastography to test the hypothesis that advancing age brings altered spatial patterns of Achilles tendon tissue elasticity that predict measured muscle tissue deformation patterns. The second aim is to predict the functional implications of age- related changes in Achilles tendon tissue mechanics on plantarflexor performance during movement. We will link measurements of human movement with a unique computational framework that includes detailed structural representations of the 3D morphology of the plantarflexor muscle-tendons and their dynamic interactions. We will test the primary hypothesis that simulating age-related changes in Achilles tendon elasticity and inter-fascicle adhesions will diminish power production and increase localized tissue strains. The third aim is to investigate age-related changes in Achilles tendon behavior during walking and its relevance to functional motor performance and response to gait interventions. We will measure in vivo Achilles tendon deformations, plantarflexor fascicle behavior, and plantarflexor power during walking. We will couple these measurements with biofeedback designed to elicit prescribed increases in plantarflexor power output. We will use these data to test the hypotheses that: 1) more uniform tendon deformations during walking with aging, which would reflect a reduction in sliding between tendon fascicles, will predict reduced ankle joint kinetics and altered plantarflexor muscle fascicle kinematics, and 2) with aging, different coordination strategies will be used to increase plantarflexor power, adaptations that will be consistent with Aim 2 model predictions. Combined, these aims will reveal the influence of age-related changes in Achilles tendon mechanics on plantarflexor muscle behavior during movement, insights critical for developing informed interventions to maintain or restore mobility while mitigating risk for muscle-tendon tissue damage.